1. Field of the Invention
The present invention relates to an electrooptic probe that couples an electrical field generated by a measured signal and an electrooptic crystal, makes light incident on this electrooptic crystal, and measures the waveform of the measured signal by the state of the polarization of the incident light. This application is based on Patent Application No. Hei 10-233351 filed in Japan, the content of which is incorporated herein by reference.
2. Description of Related Art
It is possible to couple an electrical field generated by a measured signal with an electrooptic crystal, make a laser beam incident on this electrooptic crystal, and observe the waveform of the measured signal by the state of the polarization of the laser beam. It is possible to pulse the laser beam and observe with an extremely high time resolution when sampling the measured signal. An electrooptic sampling oscilloscope uses an electrooptic probe exploiting this phenomenon.
When this electrooptic sampling oscilloscope (hereinbelow, referred to as an "EOS oscilloscope") is compared to a conventional sampling oscilloscope using an electrical probe, the following characteristics have received much attention:
1. It is easy to observe the signal because a ground wire is unnecessary. PA1 2. Because the metallic pin at the end of the electrooptic probe is isolated from the circuit system, it is possible to realize high input impedance, and as a result of this, there is almost no degradation of the state of the measured point. PA1 3. By using an optic pulse, broadband measurement up to the GHz order is possible.
The structure of a probe for an EOS oscilloscope in the conventional technology will be explained using FIG. 3. In the electrooptic probe shown in FIG. 3, a probe head 3 comprising an insulator is mounted on the end terminal of the metallic probe body 2, and a metallic pin 3a is fit into the center. Reference numeral 4 is an electrooptic element, a reflecting film 4a is provided on the end surface on the metallic pin 3a side, and is in contact with the metallic pin 3a. Reference numeral 5 is a 1/2 wavelength plate, and reference numeral 6 is a 1/4 wavelength plate. Reference numeral 7 and 8 are polarized beam splitters. Reference numeral 9 is a 1/2 wavelength plate, and reference numeral 10 is a laser diode. Reference numerals 14 and 15 are condensing lenses, and reference numerals 16 and 17 are photodiodes.
In addition, the two polarized beam splitters 7 and 8, the 1/2 wavelength plate 9, and the Faraday element 10 comprise an isolator 19 that transmits the light emitted by the laser diode 13, in order to split the light reflected by the reflecting film 4a.
Next, referring to FIG. 3, the optical path of the laser beam emitted from the laser diode 13 is explained. In FIG. 3, reference letter "A" denotes the optical path of the laser beam.
First, the laser beam emitted from the laser diode 13 is converted by the collimator lens 12 into a parallel beam that travels straight through the polarized beam splitter 8, the Faraday element 10, the 1/2 wavelength plate 9, and the polarized light beam splitter 7, and then transits the 1/4 wavelength plate 6 and the 1/2 wavelength plate 5, and is incident on the electrooptic element 4. The incident light is reflected by the reflecting film 4a formed on the end surface of the electrooptic element 4 on the side facing the metallic pin 3a.
The reflected laser beam transits the 1/2 wavelength plate 5 and the 1/4 wavelength plate 6, one part of the laser beam is reflected by the polarized light beam splitter 7, condensed by the condensing lens 14, and incident on the photodiode 16. The laser beam that has transited the polarized light beam splitter 7 is reflected by the polarized beam splitter 8, condensed by the condensing lens 15, and incident on the photodiode 17.
Moreover, the angle of rotation of the 1/2 wavelength plate 5 and the 1/4 wavelength plate 6 is adjusted so that the strength of the laser beam incident on the photodiode 16 and the photodiode 17 is uniform.
Next, using the electrooptic probe 1 shown in FIG. 3, the procedure for measuring the measured signal is explained.
When the metallic pin 3a is placed in contact with the measurement point, due to the voltage applied to the metallic pin 3a, at the electrooptic element 4 this electrical field is propagated to the electrooptic element 4, and the phenomenon of the altering of the refractive index due to the Pockels effect occurs. Thereby, the laser beam emitted from the laser diode 13 is incident on the electrooptic element 4, and when the laser beam is propagated along the electrooptic element 4, the polarization state of the beam changes. Additionally, the laser beam having this changed polarization state is reflected by the reflecting film 4a, condensed and incident on the photodiode 16 and the photodiode 17, and converted into an electrical signal.
Along with the change in the voltage at the measurement point, the change in the state of polarization by the electrooptic element 4 becomes the output difference between the photodiode 16 and the photodiode 17, and by detecting this output difference, it is possible to observe the electrical signal applied to the metallic pin 3a.
Moreover, in the above-described electrooptic probe 1, the electrical signals obtained from the photodiodes 16 and 17 are input into an electrooptic sampling oscilloscope, and processed, but instead, it is possible to connect a conventional measuring device such as a real time oscilloscope at the photodiodes 16 and 17 via a dedicated controller. Thereby, it is possible to carry out simply broadband measurement by using the Electrooptic probe 1.
However, in this electrooptic probe 1, the probe head 3 is formed by an insulator, and the probe body 2 that supports the probe head 3 is formed from metal. Due to this, the change in the electrical field of the measured signal propagates as noise to the photodiodes 16 and 17 and the laser diode 13 via the probe body 2, and there is the problem that the S/N ratio during measurement deteriorates.
In addition, in the EOS oscilloscope connected to the photodiodes 16 and 17, there are cases of using a process in which the light from the electrooptic element 4 is converted into an electric signal, is divided and used as the desired sample rate, and because frequency of the noise generated from the display of the oscilloscope is about the same as the signal frequency of the measured signal steped down to a lower frequency by sampling, this kind of noise is detected by the photodiodes 16 and 17, and there is the problem of causing deterioration of the measuring precision.